LibWeb: Factor out CanvasSettings mixin into seperate file

[Totto16]

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This refactors this mixin, that was introduced in #4506 to be the same as all other mixins, so that it can be used for #3788

This is necessary, to use CanvasSettings in #3788 . It also does the same as all other mixins, I have no plan, why it was not done like this in #4506 in the first place